1. Field of the Invention
The present invention relates to a friction stir welding apparatus for softening a portion of a plate-like member which is to be butt welded through friction heat generated by an advancing rotary tool so that the portion being softened is plastically fluidized in the rear of the advancing rotary tool, whereby the portion to be butt welded is allowed to be welded to another member when cooled and solidified.
2. Description of the Related Art
A method and apparatus which are described, respectively, in JP-W-9-508073 and JP-A-10-156557 are conventionally known as a method and apparatus for butt welding a plate-like member of relatively soft aluminum alloy. According to the principle of a friction stir welding method and apparatus of this kind, generally a steel rotary tool which is harder than the plate-like member is moved along a line extending along a portion of the plate-like member which is to be butt welded, that is, which is to be butt joined by welding (hereinafter referred to as xe2x80x9cwelding portionxe2x80x9d) while being rotationally driven so as to soften the welding portion by virtue of friction heat generated by the rotary tool so that the portion being softened is plastically fluidized in the rear of the moving rotary tool, whereby the welding portion is allowed to be welded to another member when cooled and solidified.
In the friction stir welding apparatus as described above, the rotary tool has a shank adapted to be brought into abutment with the surface of the welding portion of the plate-like member and a probe provided in such a manner as to protrude from the shank and be inserted into the welding portion for frictional rotating contact therewith. The rotary tool is formed substantially into a stepped column shape with the shank and the probe. The friction stir welding apparatus having the rotary tool so constructed normally comprises an electric motor for rotationally driving the rotary tool which is mounted thereon with a chuck and a ball screw type positioning mechanism having the electric motor mounted thereon for bringing the shank of the rotary tool into abutment with the surface of the welding portion of the plate-like member.
Incidentally, the positioning mechanism in the conventional friction stir welding apparatus has only a function to simply position the shank of the rotary tool so that the shank is brought into abutment with the surface of the welding portion of the plate-like member. Therefore, in a case where the welding portion of the plate-like member is flat and constant in thickness, the amount of insertion of the probe into the welding portion can be maintained constant. However, in a case where the welding portion is not flat or variable in thickness, the amount of insertion of the probe varies and as a result, it becomes difficult to weld the welding portion stably and accurately.
In addition, in a case where the conventional friction stir welding apparatus is mounted on an arm of a machining robot so as to weld the welding portion of the plate-like member while the probe of the friction stir welding apparatus is being moved linearly along the line of the welding portion by driving the arm, since the arm is generally driven by a plurality of servo motors, the accuracy with which the arm is re-positioned is inconstant, and it becomes difficult for the arm to hold the probe at a position at a desired height. Consequently, in this case, too, the amount of insertion of the probe into the welding portion varies and as a result, it becomes difficult to weld the welding portion stably and accurately.
Thus, an object of the invention is to provide a friction stir welding apparatus which can allow a shank of a rotary tool to follow the surface of a welding portion of a plate-like member even if the surface of the welding portion is irregular and the thickness of the same portion varies to thereby maintain constant the amount of insertion of a probe into the welding portion and which can perform a good welding operation, in particular, when mounted on an arm of a machining robot.
With a view to attaining the object, according to the invention, there is provided a friction stir welding apparatus including: a rotary tool for friction stir welding having a shank adapted to be brought into abutment with the surface of a portion of a plate-like member which is to be butt welded and a probe adapted to be inserted into the portion to be butt welded for frictionally rotating contact therewith; a rotational driving unit on which the rotary tool is mounted, the rotational driving unit rotationally driving the rotary tool; a press driving unit on which the rotational driving unit is mounted, the press driving unit pressing the shank of the rotary tool against the surface of the portion to be butt welded; a mount bracket supporting the press driving unit; a pressure sensor detecting a pressing force exerted on the surface of the portion to be butt welded by the shank of the rotary tool; and a control unit controlling the driving of the press driving unit such that the pressing force falls within a predetermined range based on a detection signal sent from the pressure sensor.
In the friction stir welding apparatus according to the invention, the rotational driving means rotationally drives the rotary tool, and the press driving means inserts the probe of the rotary tool into the welding portion of the plate-like member so that the shank of the rotary tool is pressed against the surface of the welding portion. In this state, when the friction stir welding apparatus moves relative to the plate-like member linearly along the line of the welding portion, the probe of the rotary tool is brought into a frictional rotating contact with the welding portion to thereby soften the same portion by virtue of friction heat generated by the frictional rotating contact, whereby the portion being softened starts to be plastically fluidized in the rear of the moving probe, and the welding portion is butt welded when cooled and solidified.
As this occurs, in the friction stir welding apparatus according to the invention, the pressure sensor detects the pressing force of the shank of the rotary tool relative to the surface of the welding portion of the plate-like member, and the control means controls the driving of the press driving means so that the pressing force of the shank falls within the predetermined range based on the detection signal from the pressure sensor. Consequently, the shank of the rotary tool can follow the surface of the welding portion even if the surface of the welding portion of the plate-like member has irregularities or the same portion varies in thickness, whereby the amount of insertion of the probe into the welding portion can maintain constant.
In the friction stir welding apparatus according to the invention, in a case where the mount bracket is constructed so as to be mounted on an arm of a machining robot, the welding portion can be butt welded by mounting the mount bracket on the arm of the machining robot and driving the arm so that the probe of the rotary tool moves along the welding portion of the plate-like member. As this occurs, since the amount of insertion of the probe of the rotary tool into the welding portion of the plate-like member is maintained constant, even if the accuracy with which the arm of the machining robot is re-positioned is inconstant, the machining robot only has to drive the arm linearly along the line of the welding portion.
In the friction stir welding apparatus according to the invention, in a case where there is provided a mount plate for fixing the rotational driving means to the press driving means for imparting the pressing force to the shank of the rotary tool, the pressure sensor can be held between the mount plate and the rotational driving means. In this case, it is preferable to use a piezoresistant element or a piezoelectric polymer as the pressure sensor.
In addition, in the friction stir welding apparatus according to the invention, while the rotational driving means normally uses an electric motor as a driving source, a hydraulic motor may be used as the driving source. Additionally, while the press driving means can be constituted by a ball screw mechanism adapted to be driven by the electric motor, the press driving means may be constituted by a linear actuator, a pneumatic cylinder, a hydraulic cylinder or the like.